(1) Field of the Invention
This invention relates to inspection of thermal barrier coatings, and more particularly to inspection of coatings on turbine components.
(2) Description of the Related Art
Gas turbine engine components (e.g., blades, vanes, seals, combustor panels, and the like) are commonly formed of nickel- or cobalt-based superalloys. Desired operating temperatures often exceed that possible for the alloys alone. Thermal barrier coatings (TBCs) are in common use on such components to permit use at elevated temperatures. Various coating compositions (e.g., ceramics) and various coating methods (e.g., electron beam physical vapor deposition (EBPVD) and plasma spray deposition) are known.
An exemplary modern coating system is applied to the superalloy substrate by an EB-PVD technique. An exemplary coating system includes a metallic bondcoat layer (e.g., an overlay of NiCoCrAlY alloy or diffusion aluminide) atop the substrate. A thermally insulating ceramic top coat layer (e.g., zirconia stablized with yttria) is deposited atop the bondcoat. During this deposition, a thermally grown oxide layer (TGO), e.g., alumina, forms on the bondcoat and intervenes between the remaining underlying portion of the bondcoat and the top coat.
The coatings are subject to potential defects. For example, the TGO to bondcoat interface tends to suffer from separations/delaminations. Such defects tend to be inherent, so threshold degrees of defect may determine the utility of a given component. Defects may also form during use.
Much of existing inspection involves destructive testing used to approve or reject batches of components. Exemplary destructive testing involves epoxy-mounting and sectioning a component followed by microscopic examination. The TGO is a critical element. This may be viewed via scanning electron microscope (SEM) at 1,000× or higher. Quality standards are used to approve or reject the batch based upon visual interpretation of the SEM images.
Destructive testing suffers from many general drawbacks as do its various particular techniques. The former includes the cost of destroyed components, the inaccuracy inherent in batch sampling, and the cost of time. U.S. Pat. No. 6,352,406 discloses an alternate system involving coating of a pre-couponed turbine blade facsimile in lieu of cutting an actual blade. This may slightly reduce the time spent, but does not address the fundamental problems of destructive testing.
Laser fluorescence has been used for nondestructive evaluation of limited coating parameters. In one example, the beam of a ruby laser is shined on the ceramic top coat and passes therethrough to reach the TGO. The TGO fluoresces and the emitted light passes through the top coat to a sensor. Characteristics of the flurorecence indicate stress in the TGO. Separation/delamination voids are associated with reduced stress and can thus be detected. U.S. Pat. No. 6,072,568 discloses such inspection.
There remains a substantial need for improvement in testing techniques.